The present invention relates generally to ink jet printing systems and, more particularly, to a molded charge electrode structure or charge plate and method of fabricating the charge plate for use in an ink jet printing system.
In ink jet printers, tiny ink drops are selectively deposited onto a medium, typically a moving web or sheet to form a printed image. The ink drops are formed from liquid filaments emerging from tiny openings in an orifice plate communicating with an ink reservoir containing pressurized electrically conductive ink. By mechanically stimulating the orifice plate, uniformly sized and spaced drops are forced to break off from the fluid filaments.
As each drop separates from its associated fluid filament, it is permitted to remain uncharged or is selectively charged to one or more charge levels by an associated charge electrode. The drops then pass through an electrical deflection field.
Printing is performed in a variety of ways. For example, charged drops can be deflected by the deflection field to a drop catcher and uncharged drops which are not deflected by the field continue past the catcher to form the printed images, for example, on a moving web of material. Alternatively, the uncharged drops may be caught with the charged drops being deflected to one or a variety of positions on the moving web dependent upon the charge level of the particular drop. In any event, the importance of the charge electrode structure to proper operation of an ink jet printer is clearly evident.
Typical prior art charge electrodes have been formed by coating electrically conductive material onto a nonconductive substrates. Many prior art charge electrodes have partially or wholly surrounded the corresponding ink jet stream and normally extend uniformly along the stream for a distance of at least several drop diameters. Due to the tendency of the ink drops to separate from the filaments at different points, the electric field produced by a charge electrode must be uniform along the length of the ink filaments so that drops are properly charged regardless of their exact point of separation.
Early patents to Loughren, U.S. Pat. No. 3,404,221 and to Sweet et al, U.S. Pat. No. 3,373,437, utilized cylindrically shaped tubular electrodes to completely surround the ink jet stream or U-shaped channel electrodes to partially surround the stream. Unfortunately, accurate placement of the tubes or channels into a support structure and electrical connection of such electrodes to a signal source are both difficult and time consuming. Such placement and connection problems increase as the spacing between electrodes is reduced in multiple jet systems, utilizing hundreds of individual streams of ink drops spaced within a few thousandths of an inch of one another. Also, as the spacing between adjacent electrodes is reduced, the tube or channel walls become extremely thin. Thin electrode walls reduce reliability and, at some point, preclude formation of such electrodes.
Various attempts have been made to reduce the difficulty and expense of forming charge electrodes. For example, Beam et al, U.S. Pat. No. 3,586,907, shows a charge ring plate wherein a series of holes have an electrically conductive coating surrounding each hole and extending along the walls to form the charge electrodes. Electrical lead lines are plated onto the surface of the charge plate and extend each charge ring to a connection point. The techniques involved in plating the walls of the holes to obtain a continuous and uniform coating are complex and involve plating in several dimensions. Similarly, coating the U-shaped channel shown in Culp, U.S. Pat. No. 3,618,858, with an electrically conductive material also involves plating in several dimensions.
Robertson, U.S. Pat. Nos. 3,604,980 and 3,656,171, disclose charge electrodes formed by a series of strips of electrically conductive material plated onto a dielectric planar surface with each strip connected to a charging signal source.
Kenworthy, U.S. Pat. No. 4,223,321, discloses a planar charge plate wherein grooves are cut into the edge of a nonconductive substrate with the grooves and lands being metalized and printed circuit leads formed leading from the grooves. The grooves are then filled with an electrically conductive material, such as solder, and the front face of the structure is lapped to remove excess solder and metal plating from the lands to form the charge plate.
The charge electrodes of Robertson and Kenworthy differ from other prior art charge electrodes in that they do not surround or partially surround the drop streams. However, the formation of Robertson's conductive strips still requires plating in several dimensions and Kenworthy requires accurate machining initially to form the grooves in the substrate.
Bahl and Schutrum, U.S. Pat. No. 4,347,522, discloses a method of forming charge electrodes by laminating a plurality of thin sheets of electrically conductive material to construct essentially any form of electrode.
Another example of forming charge electrodes is disclosed by Bahl et al in U.S. patent application Ser. No. 348,476, filed on Feb. 12, 1982, now U.S. Pat. No. 4,419,674. In this application, wires are wrapped around a nonconductive charge plate support structure, preferably having wire locating notches formed into opposite end faces. The wire is then potted up to adhere it to the structure and the wire along one end face is exposed to form individual charge electrodes. The wire along the adjacent face is left embedded, except at its very end, so as to secure the wire to the structure.
While charge electrodes manufactured in accordance with the teachings of a variety of the prior art provide satisfactory operation of ink jet printers and have typically provided somewhat reduced costs as the art has advanced, more simplified methods for producing reliable charge electrode structures at still further reduced costs are always needed and desirable for advances and improvements in the ink jet art.